1. Field of the Invention
The present invention relates in general to well-logging operations, and is particularly useful in anisotropic formations.
2. Description of the Prior Art
In oil and gas exploration, measurement-while-drilling or wireline systems are utilized to develop logs of the earth formations surrounding the borehole. In general, logging packages include: a transmission system that energizes the earth formations with either an electromagnetic field or currents, and at least one receiver system for monitoring the effect that the earth formations (and borehole) have on the field or current.
The electrical properties of the earth formations provide information about the geologic materials that make up the formations, and about their likely oil, gas, and water content. The most useful electrical attribute is resistivity (or conductivity). However, the dielectric property of the earth formations is also of interest.
Some earth formations are isotropic, which means that the earth formation does not preferentially conduct electricity in any particular direction. Other formations are anisotropic, which means that the earth formation may have a greater conductivity in one particular direction than in another particular direction. This is particularly true in many sedimentary geologic formations in which current flows more easily in the direction parallel to the bedding planes than transversely to them due to the fact that a number of mineral crystals are flat or elongated in shape, and they naturally took an orientation parallel to the plane of sedimentation when they were deposited.
In oil and gas exploration, the anisotropy of earth formations can be expressed as a horizontal conductivity, and a vertical conductivity which is transverse. Alternatively, an anisotropy coefficient (.lambda.) can be utilized to quantify the anisotropy in accordance with Equation No. 1: